1. Field of the Invention
The present invention relates to an electrophoretic display unit which utilizes migration of charged electrophoretic particles for displaying an image, and to a method for driving the display unit.
2. Related Background Art
In recent years, with remarkable progress of digital techniques, the amount of information which individual persons can deal with is dramatically increasing. With this technical progress, display units are being developed for less power consumption and a smaller thickness of the unit. Of the display units, liquid crystal display units can meet the above needs, and are developed energetically and have been commercialized. However, conventional liquid crystal display units have problems such that the displayed letters may be not readily readable depending on the viewing angle or light reflection, and heavy visual load due to the flickering or insufficient brightness of a light source. Such problems are not solved yet. Therefore, reflection type display units are expected to be promising.
One example is an electrophoretic display unit disclosed by Harold D. Lees et al. (U.S. Pat. No. 3,612,758).
FIG. 27A illustrates an example of the structure of the electrophoretic display unit. This kind of electrophoretic unit has a pair of substrates 1a, 1b counterposed with a prescribed interspace, an insulating liquid filled between the substrates 1a, 1b, many colored charged electrophoretic particles 3 dispersed in the insulating liquid 2, and display electrodes 15a, 15b placed respectively on the substrates 1a, 1b of the respective pixels. Partitioning walls 7 are provided between the pixels to prevent migration of colored charged electrophoretic particles 3 to other pixels to enable uniform display. In this display unit, colored charged electrophoretic particles 3, which are charged positively or negatively, are adsorbed onto the display electrode 15a or 15b depending on the polarity of the voltage applied to display electrodes 15a, 15b. Insulating liquid 2 and colored charged electrophoretic particles 3 have different colors. With colored electrophoretic particles 3 adsorbed onto display electrode 15a at the observer side, the color of particles 3 is visually recognizable (FIG. 27B), whereas, with colored electrophoretic particles 3 adsorbed onto display electrode 15b at the other side, the color of insulating liquid 2 is visually recognizable (FIG. 27A). Therefore, various images can be displayed by controlling the polarity of the applied voltage for each of the pixels. This type of display unit is classified herein as a xe2x80x9cvertical migration typexe2x80x9d.
However, in the vertical migration type electrophoretic device, insulating liquid 2 should contain a colorant such as a dye or an ionic substance. The colorant contained is liable to cause instability in the electrophoresis operation by additional charge transfer, which may lower the performance, life, or stability of the display unit.
To solve such problems, electrophoretic display units shown in FIGS. 28A and 28B are disclosed in Japanese Patent Application Laid-Open Nos. 49-5598, 49-024695, and 11-202804 (hereinafter referred to as xe2x80x9chorizontal migration type electrophoretic display unitsxe2x80x9d). Such a horizontal migration type electrophoretic display unit has also a pair of substrates 1a, 1b, insulating liquid 2 filled between the substrates 1a, 1b, many colored charged electrophoretic particles 3 dispersed in the insulating liquid 2, and a pair of display electrodes 25a, 25b for each of the pixels. However, the pair of display electrodes 25a, 25b are not placed to hold insulating liquid 2 therebetween unlike the aforementioned type, but are placed on one substrate 1b. In such a horizontal migration type electrophoretic display unit, insulating liquid 2 should be transparent but need not contain a colorant, thereby avoiding the aforementioned problems. In this display unit, one display electrode 25a is coated with a colored layer having the same color as charged electrophoretic particles 3 (e.g., black), and the other display electrode 25b is coated with another color (e.g., white). The colored electrophoretic particles 3 migrate horizontally (in the direction parallel to the substrate) in accordance with the polarity applied to display electrodes 25a, 25b, and are adsorbed by display electrode 25a or 25b. With charged electrophoretic particles 3 adsorbed by display electrode 25a, the color of display electrode 25b is visually recognizable more readily (FIG. 28A), whereas with charged electrophoretic particles 3 adsorbed by display electrode 25b, the entire pixel is visually recognizable to have the color of electrophoretic particles 3 (FIG. 28B). Accordingly, various images can be displayed by controlling the polarity of the applied voltage for each of the pixels.
The systems for electrical addressing the display unit having the pixels arranged in a matrix are roughly classified into two systems: active matrix systems and simple matrix systems.
In the active matrix system, a switching element such as a thin film transistor (TFT) is formed in each of the pixels, and the voltage applied to the pixel is controlled independently for each of the pixels. With this system, the horizontal migration type electrophoretic display unit can be driven with a high display contrast. However, this active matrix system has disadvantages of a high cost for processing, and difficulty in formation of thin film transistor on a polymer substrate owing to the high process temperature. These disadvantages are serious especially in formation of a paper-like display for low-cost flexible display. For offsetting such disadvantages, are proposed a process for thin film transistor by use of a polymer material applicable to a printing process, and a process of TFT transfer system which does not need heating of the substrate. However, the possibility of commercialization is not sure.
On the other hand, in the simple matrix system, the necessary constitutional elements for addressing are X-Y electrode lines only. Therefore, this system is of low cost and can be formed on a polymer substrate. For application of a writing voltage to a selected pixel, the voltage for writing is applied to the X electrode line and the Y electrode line crossing at the selected pixels. However, if the electrophoretic display unit is driven by the simple matrix system, the writing can be made also in a part of neighboring pixels around the selected pixels (so-called crosstalk phenomenon) to lower the display contrast significantly. This is caused necessarily, because the electrophoretic display unit does not have a precise threshold to the writing voltage.
To solve this problem in the electrophoretic display having no threshold voltage in principle, a method is disclosed in which the simple matrix is driven by employing a control electrode in addition to the pair of electrodes as a three-electrode system.
Most of the disclosures on the three-electrode system relate to vertical migration type electrophoretic display unit such as the one disclosed in Japanese Patent Publication No. 61-016074 (U.S. Pat. No. 4,203,106).
The three-electrode system for the horizontal migration type electrophoretic display unit is disclosed only in Japanese Patent Gazette No. 02,740,048 (U.S. Pat. No. 5,345,251, Japanese Patent Application International Publication No. 8-507154). In the disclosed system, according to the above Patent Gazette, insulating liquid 2 employed is considered to be not transparent, but to be colored. Therefore, this system is different from the ones of the horizontal migration type electrophoretic display unit disclosed in the aforementioned Japanese Patent Laid-Open Nos. 49-5598 and 11-202804, and the display unit of the present invention in which the insulating liquid is transparent.
Japanese Patent Gazette No. 02,740,048 discloses two constitutions (a first constitution and a second constitution) for the arrangement of the control electrode. In the first constitution, the control electrode (grid line) is placed, as indicated by symbol 36a in FIG. 29A, on rear plate 1a counterposed to face plate 1b with an interspace of 25-116 xcexcm. In FIG. 29A, the symbols indicate as follows: 35a, a cathode element; 35b, an anode element; 37, a chromium layer formed on the anode element; and 38, a photoresist formed on the chromium layer. Chromium layer 37 and photoresist 38 gives a level difference of about 0.3 xcexcm at the boundary between cathode element 35a and anode element 35b. 
In the second constitution, the control electrode (grid line) is placed, as indicated by symbol 36b in FIG. 29B, between cathode element 35a and anode element 35b on face plate 1b. 
In any of the first constitution and the second constitution, fork-shaped cathode element 35a which is an assemblage of plural line electrodes, and fork-shaped anode element 35b which is an assemblage of plural line electrodes placed between the lines of cathode element 35a are placed on face plate 1b (see FIG. 30). In FIG. 30, cathode element 35a and anode element 35b are shown to be constituted respectively of one line for convenience of explanation.
The basic operation of the electrophoretic display unit (first constitution) shown in FIG. 29A is explained by reference to FIGS. 31A to 31C. In this case, charged electrophoretic particles 3 employed have a yellow color and are charged negatively.
On application of voltages of 0V to grid line 36a, 0V to anode element 35b, and about +12V to cathode element 35a, charged electrophoretic particles 3 migrate onto the surface of cathode element 35a to give a yellow display state of the pixel (FIG. 31A).
Then, on application of voltages of 0V to grid line 36a, +15V to anode element 35b, and 0V to cathode element 35a, charged electrophoretic particles 3 migrate to cover anode element 35b (FIG. 31B).
In the state shown in FIG. 31A, charged electrophoretic particles 3 are prevented from migration to anode element 35b by applying a negative voltage to grid line 36a as the write-inhibiting voltage (FIG. 31C).
On the other hand, in the second constitution shown in FIG. 29B, the migration of charged electrophoretic particles 3 between cathode element 35a and anode element 35b (writing in the pixel) can be inhibited by applying a write-inhibiting voltage to grid line 36b. In the pixel to which the write-inhibiting voltage is not applied, writing can be conducted by migration of charged electrophoretic particles 3 in accordance with the voltage applied to cathode element 35a and anode element 35b. In this second constitution, all of the constitutional elements for driving 35a, 35b including grid line 36b are placed on one and the same plate 1b, so that the registration can be simplified in the bonding of two plates 1a, 1b. 
The horizontal migration type electrophoretic devices of the first constitution and the second constitution, however, have problems as shown below. FIGS. 32A and 32B and FIGS. 33A to 33C are drawings for explaining respectively the second constitution and the first constitution.
(1) Occurrence of Cross Talk
Cathode element 35a and anode element 35b extend on the first substrate in a direction perpendicular to the drawing sheet face to constitute line electrodes, and grid lines 36 are formed on the second substrate in the direction perpendicular to the line electrodes to form row electrodes.
In the first constitution, when different voltages are applied to the adjacent grid lines (control electrodes) 36a, 36axe2x80x2 (in FIGS. 33A to 33C, grid line 36axe2x80x2 is superposed on grid line 36a and is not shown), an interaction of the electric fields may be caused in the space including the adjacent grid lines 36a, 36b, disadvantageously. This is explained below in more detail. When a display voltage is applied to cathode element 35a and anode element 35b, a retaining voltage is applied to one 36a of the two adjacent grid lines to inhibit the migration of charged electrophoretic particles 3, and no retaining voltage is applied to the other grid line 36axe2x80x2, then charged electrophoretic particles 3 are expected to migrate smoothly in the pixel to which no retaining voltage is applied. However, unexpectedly, the charged electrophoretic particles 3 do not migrate smoothly owing to the influence of the retaining voltage-applied grid line 36axe2x80x2. To solve the problem, one method is to lower the control voltage. However, this decreases the retention effect (inhibition of migration of charged electrophoretic particles 3) to cause cross talk.
(2) Increase of Power Consumption
In inhibition of migration of charged electrophoretic particles 3 by voltage application in an electrophoretic display unit having a less level difference, the particle migration as shown by symbol G2 in FIG. 33B can be prevented by increasing sufficiently the voltage applied to grid line 36a. However, the increase of the applied voltage causes other problems that power consumption increases, and the operation state of the charged electrophoretic particles 3 is unstable owing to the unintended electric field produced by remaining charge of the charge injected by high voltage into the insulating member in the element.
(3) Limitation in Display Contrast
In the first constitution, excessive level difference may retard the jump of the charged electrophoretic particles 3, even with application of a voltage between cathode element 35a and anode element 35b, leaving a part of the charged electrophoretic particles 3 in the bottom level (see G1 in FIG. 33A) to decrease the display contrast by insufficient number of the particles for covering anode element 35b. For solving the above problem, one method is to make the level difference to be not excessively high (nearly equal to the diameter of charged electrophoretic particles 3). However, with this method, the effect of the level difference in retardation of migration of charged electrophoretic particles is insufficient. Therefore, even if a voltage is applied to grid line 36a as shown in FIG. 33B to retard the migration of charged electrophoretic particles 3, a part of charged electrophoretic particles 3 can migrate over the level difference as shown by symbol G2 to cause cross talk phenomenon to lower the display contrast. This is a serious problem.
As described above, the decrease of the level difference may make the capacity of the hollow portion (the lower portion of cathode element 35a) insufficient for holding the entire charged electrophoretic particles 3 as shown by G3 in FIG. 33C and may cause overflow of the particles toward anode element 35b, resulting in decrease of the display contrast disadvantageously.
Another method to solve the problem with the decreased level difference is increase of the area of cathode element 35a to increase the capacity of the hollow. However, this method makes smaller the area ratio of the cathode element to the anode element, decreasing the display contrast disadvantageously.
In the electrophoretic display unit disclosed in Japanese Patent Application International Publication No. 8-507154, the display contrast is limited also by the electrode construction. As shown in FIG. 30, in the display unit, fork-shaped cathode element 35a and fork-shaped anode element 35b are placed in counterposition in the same level on face plate 1b. For the insulation, the elements should be separated at a certain distance. The separation decreases necessarily the element area (occupation area of cathode element 35a and anode element 35b) in one pixel, resulting in decrease of the display contrast disadvantageously.
(4) Difficulty in Production of Fine Display Unit
In the construction in which the fork-shaped elements 35a, 35b are placed in one and the same plane, finer display portion requires fineness of the respective element and smaller gap between the elements. Therefore the electric short circuit is liable to occur between the elements, making production of the fine display unit difficult.
(5) Limitation to One-Way Writing
In the first constitution disclosed in Japanese Patent Application International Publication No. 8-507154, the retardation of migration of the charged electrophoretic particles by the level difference is limited to the migration from the lower level to the higher level. The migration from the higher level to the lower level is rather accelerated. Accordingly, the driving method is limited to one-way writing, in which entire electrophoretic particles 3 in the picture face are collected to the lower level for resetting entirely, and thereafter one-way writing is conducted. Neither two-way writing nor selective partial rewriting of the picture image can be practiced with this constitution, disadvantageously.
The electrophoretic display unit can be constituted by employing plates 1a, 1b of a flexible material to make the units foldable, but the interval between rear plate 1a and face plate 1b cannot readily be kept precise. In the first constitution, since grid line 36a, cathode element 35a, etc. are formed respectively on different plates 1a, 1b, the interspace is liable to vary, lowering the controllability, disadvantageously. Further, in bonding of plate 1a and plate 1b, since grid line 36a, cathode element 35a, etc. are formed respectively on separate plates 1a, 1b, precise registration is required to counterpose grid line 36a and cathode element 35a to each other, disadvantageously.
On the other hand, in the second constitution, grid line 36b, cathode element 35a, etc. are placed on one and the same plate 1b, so that the above problems are not caused. A display unit of high resolution can be realized by use of a flexible plastic plate as the substrate. Further, the plate bonding operation can be simplified.
In the second constitution, however, even though migration of electrophoretic particles 3 between cathode element 35a and anode element 35b is inhibited, electrophoretic particles 3 migrate to be apart from grid line 36 to cause nonuniform distribution, not distributed uniformly on the surface of cathode element 35a or anode element 35b to cause significant lowering of the display contrast disadvantageously. Further as shown in FIGS. 32A and 32B, the electrophoretic particles having migrated once near to rear plate 1a are not released from rear plate 1a by simply changing the polarity of the voltage applied to grid line 36b, cathode electrode 35a, and the like, making the control impossible.
Accordingly the present invention intends provide an electrophoretic display unit and a driving method thereof without the aforementioned disadvantages.
For achieving the above objects, the present invention provides an electrophoretic display unit, which has a first substrate and a second substrate conterposed with an interspace, an insulating liquid placed in the interspace, and colored charged electrophoretic particles dispersed in the insulating liquid, and having a stage formed in the interspace along the second substrate, the stage giving a first surface facing a thicker layer portion of the insulating liquid, a second surface facing a thinner layer portion of the insulating liquid on the second substrate, and a side wall surface of the stage connecting the first surface and the second surface, a first display electrode being placed along the first surface, and a second display electrode being placed along the second surface, wherein a third electrode is provided along an intersection line of the side wall surface and an imaginary plane placed at a prescribed distance from the second surface and nearer to the first surface than the second surface.
In the present invention, the third electrode is preferably placed on the aforementioned imaginary plane, and at least a part of the third electrode is overlaid with the second electrode viewed perpendicularly from the second substrate.
In the present invention, the entire third electrode is overlaid with the second electrode, or the third electrode has a portion not overlaid with the second electrode, the portion occupying a boundary area between the area of the first electrode and the area of the second electrode viewed perpendicularly from the second substrate.
The electrophoretic display units of the present invention comprise a control means for controlling the voltage applied to the third electrode, and the state of the display is switched, during the time in which the voltages are applied to the first electrode and the second electrode to be capable of causing migration of the electrophoretic particles, by controlling the potential of the third electrode to be intermediate between the potential of the first electrode and the potential of the second electrode to allow the charged electrophoretic particles to migrate, and controlling the potential of the third electrode to be higher or lower than the potentials of both the first electrode and the second electrode depending on the polarity of the charged electrophoretic particles to inhibit the migration of the charged electrophoretic particles.
The present invention provides also a driving method for driving the electrophoretic display unit wherein a process for switching the display state comprises a first step of impelling the charged electrophoretic particles from one of the first electrode and the second electrode toward the third electrode, and a second step of impelling the charged electrophoretic particles having migrated to the vicinity of the third electrode in the first step to the opposite one of the first electrode and the second electrode.
In the aforementioned switching process, the potential of the third electrode in the first step is set to be intermediate between the potential of the first electrode and the potential of the second electrode, and the potential of the third electrode in the second step is set to be higher or lower than the potential of the third electrode in the first step, depending on the polarity of the charged electrophoretic particles.
The present invention provides a driving method for driving the electrophoretic display unit having a second control electrode on the second substrate, wherein a process for switching the display state comprises a first step of impelling the charged electrophoretic particles from one of the first electrode and the second electrode toward the third electrode, and a second step of impelling the charged electrophoretic particles having migrated to the vicinity of the third electrode in the first step to the opposite one of the first electrode and the second electrode; wherein the potential of the third electrode in the first step is set to be intermediate between the potential of the first electrode and the potential of the second electrode, the potential of the third electrode in the second step is set to be higher or lower than the potential of the third electrode in the first step, depending on the polarity of the charged electrophoretic particles, and the potential of the fourth electrode is set to be intermediate between the potential of the first electrode and the potential of the second electrode.